This invention relates to an electronic musical instrument, and more particularly to an electronic musical instrument using a digital signal processing system in which the value of an amplitude at a sampled point of a desired waveform of one period of musical tone is calculated and a musical tone is generated by reading out the thus calculated sampled value with a note-clock signal having a frequency related to the desired pitch of a musical tone.
In the prior art, for example in U.S. Pat. Nos. 3,515,792 and 3,610,799, a digital organ has been proposed in which one period of the waveform of a musical tone for each tablet is sampled and stored in a Read Only Memory (ROM) in a digital-form. A musical tone is generated by reading out the stored data corresponding to the amplitude of the waveform at a sampled point with the note-clock signal having a repetitive frequency related to the pitch of the tone of the actuated key. As only one waveform stored in the ROM is prepared for each of the tone tablet switches, when several tone tablets are selected and assigned, two or more ROMs are accessed at the same time and their outputs are combined.
In the digital organ of the above system, when two or more keys are actuated simultaneously, as the ROM of the desired waveforms must be read out by two or more note-clock pulses with different repetitive frequencies, there is caused a read-out error at a coincident timing of the plural note-clock pulses. For avoiding this read-out error, each note-clock pulse should be prepared so as not to have any coincident timing, and moreover the ROM should be read out while being multiplexed in time by plural note clock pulses. Therefore, a complicated system is required for processing signals in complex form at a high speed in this prior art digital organ. Furthermore, the musical tone signal generated in digital form from the ROM is reshaped to have the desired envelope such as an attack, a rise or a decay, or a fall off of the signal. Such a processing for forming the envelope of the musical tone signal is also performed in digital form, and therefore the staircase changes of the amplitude of the signal inevitably results in a tone signal which has a disadvantage from the standpoint of a natural feeling, especially in the case of a very long decay time i.e., a so-called sustain. Moreover, such a digital organ has a further disadvantage that custom-made ICs or LSIs are required to build the organ.